Obesity is one of the most prevalent health conditions with about 30% of the world's adult population being either overweight or obese, causing an increased risk for cardiovascular diseases, diabetes, and certain types of cancer. (Ng, M., Fleming, T., Robinson, M., Thomson, B., Graetz, N., Margono, C., Mullany, E. C., Biryukov, S., Abbafati, C., Abera, S. F., et al.: Global, regional, and national prevalence of overweight and obesity in children and adults during 1980-2013: a systematic analysis for the global burden of disease study 2013. The Lancet 384(9945), 766-781 (2014)) Central obesity, also known as abdominal obesity, is the excessive build-up of fat around stomach and abdomen. Central obesity has been held responsible for high levels of LDL cholesterol and triglycerides and lower levels of HDL cholesterol.
Obesity contributes to increased heart disease, diabetes, and the likelihood of over 65 other diseases. (Wang, A, Kinsinger, L S, Kahwati, L C, Das, S R, Gizlice, Z, Harvey, R T, Burdick, M B & Yevich, S J. Obesity and weight control practices in 2000 among veterans using V A facilities. Obes Res 13, (2005)).
Human adult stem cells can regenerate the cellular population via self-renewal and their ability to differentiate and maintain asymmetric cellular division. The hematopoietic and mesenchymal stem cells were predominantly characterized and studied for stem cell markers and differentiation into specific lineages. Since then, it is now recognized that stem cells reside within stem cell niches of the adult organ where the number of stem cells are regulated. Amongst these, the adipose-derived stem cells (ASCs) show great therapeutic potential in regenerative medicine as they are relatively easy to isolate and are obtained at higher yields.
Human ASCs reside within the stromal vascular fraction (SVF) of the adipose tissue. ASC maintain a quiescent stage in vivo and can undergo differentiation into adipocytes, osteoblasts or chondrocytes (Carter G, Apostolatos A, Patel R, et al. Dysregulated Alternative Splicing Pattern of PKCδ during Differentiation of Human Preadipocytes Represents Distinct Differences between Lean and Obese Adipocytes. ISRN Obes 2013, 2013:161345; Watson J E, Patel N A, Carter G, et al. Comparison of Markers and Functional Attributes of Human Adipose-Derived Stem Cells and Dedifferentiated Adipocyte Cells from Subcutaneous Fat of an Obese Diabetic Donor. Adv Wound Care (New Rochelle) 2014, 3:219-228). ASC maintain their self-renewal ability by expressing transcriptional factors that regulate its ability to differentiate. Tissue damage and diseases often cause the ASC to undergo hyperproliferation leading to dysfunctional cells. It was previously shown that the ASC obtained from obese patients have distinct differences in their genetic profiles during differentiation to adipocytes compared to lean (Carter 2013). In addition, it is known that depot-specific differences are prominent in white adipose tissue. For example, leptin is produced predominantly by subcutaneous adipose tissue; adiponectin is expressed higher in omental adipose tissue.
Excess omental fat is central to increased risk factor for cardiovascular diseases and diabetes mellitus. Protein kinase C is a family of serine/threonine kinases with 11 isoforms. The primary amino acid structure of PKCs can be divided into conserved regions (C1-C4) separated by the variable regions (V1-V5). All PKCs have an N-terminal regulatory domain and a C-terminal catalytic domain separated by the V3 hinge region. The protein kinase C family is subdivided into three groups based upon their activation by calcium, phosphatidyl serine, diacyl glycerol or phorbol esters: classical or conventional PKCs (α, βI, βII and γ), novel PKCs (δ, ε, η and θ) and atypical PKCs (ζ, λ/ι). PKCs are also activated by proteolytic cleavage at the V3 hinge region by calpain I, II or caspase-3 to generate a constitutively active catalytic domain of PKC.
PKCδ, a novel PKC, plays an important role in cellular differentiation, proliferation and apoptosis. In addition, several reports have indicated the role of PKCδ in stem cell differentiation (Hamdorf M, Berger A, Schüle S, et al. PKCδ-induced PU.1 phosphorylation promotes hematopoietic stem cell differentiation to dendritic cells. Stem Cells 2011, 29:297-306; Lee H J, Jeong C H, Cha J H, et al. PKC-delta inhibitors sustain self-renewal of mouse embryonic stem cells under hypoxia in vitro. Exp Mol Med 2010, 42:294-301). PKCδ is alternatively spliced to generate PKCδI and PKCδVIII variants in humans (Patel N A, Song S S, Cooper D R. PKCdelta alternatively spliced isoforms modulate cellular apoptosis in retinoic acid-induced differentiation of human NT2 cells and mouse embryonic stem cells. Gene Expr 2006, 13:73-84; Jiang K, Apostolatos A H, Ghansah T, et al. Identification of a novel antiapoptotic human protein kinase C delta isoform, PKCδVIII in NT2 cells. Biochemistry 2008, 47:787-97). The inventors have previously shown the expression of PKCδ splice variants in the ASC as well as the role of PKCδVIII in regulating hTERT in senescence (Carter G, Patel R, Apostolatos A, et al. Protein kinase C delta (PKCδ) splice variant modulates senescence via hTERT in adipose-derived stem cells. Stem Cell Investig 2014, 1:3).
The inventors have investigated the differences in stem cells derived from subcutaneous and omental adipose depots from a lean and an obese donor. Stem cell markers, exosomes and cellular senescence in the ASC isolated from different depots were compared and the role of PKCδ in ASC niche was evaluated.